Lithography processes constitute a significant process step in the semiconductor industry. In conventional lithographic methods, structures of a mask are transferred in the ratio 1:1 or in demagnified fashion, e.g. in the ratio 4:1, onto a light-sensitive polymer layer (photoresist) that has been applied to an (if appropriate patterned) semiconductor wafer. The wavelength ranges used in this case may lie for example in the visible range, in the DUV (deep ultraviolet) range or in the soft X-ray range, also called EUV (extreme ultraviolet). The photoresists used are often so-called positive resists, which, after exposure with photons of specific wavelengths, become soluble in suitable developers and are thereby removed at the exposed locations during the development process. However, it is also possible for negative resistance to be used, in the case of which unexposed regions of the photoresist are stripped away and exposed regions are retained.
The optimum exposure dose, that is to say the optimum radiation power impinging on the photoresist per unit area over a specific period of time, is of crucial importance in order that the structure transfer from the mask into the photoresist is effected with maximal dimensional fidelity and the resist profiles after development are as steep as possible. Underexposure can lead to incomplete removal of the resist in the exposed regions. The resist sidewalls may slope to shallowly and be unsuitable for a subsequent dimensionally accurate structure transfer from the resist into the underlying layer or the substrate by means of plasma etching or ion implantation. Overexposure can lead to an expansion of the exposed regions and thus to undesirably narrow resist webs.
In the prior art, in order to determine the correct exposure dose for each batch of wafers, a so-called precursor wafer or test wafer is exposed with different doses (exposure gradation), developed and measured for structural accuracy in an inline CD measuring unit. With the “ideal” exposure dose determined therefrom, the wafers of the batch are exposed uniformly. Fluctuations in the resist sensitivity, the mean resist thickness, the resist support, etc. from wafer batch to wafer batch are taken into account in this way. However, the method is time-consuming, lowers the throughput of the exposure unit, which cannot be utilized during the processing of the test wafer, and thereby causes increased costs. This method does not register variations in the resist thickness over the wafer and from wafer to wafer and dose fluctuations which are caused by the exposure unit during the exposure of a batch.
In the case of the conventional optical steppers and scanners in the visible range and in the DUV range, portions of the light are coupled out by means of suitable components, such as, for example, beam splitters, etc., and measured in real time. Using electronic control mechanisms, temporal fluctuations in the exposure power are then compensated for by means of shutters, diaphragms and/or the speed of the scanning tables.
Such beam splitters are not possible in the EUV range since the light absorption of the materials is too great for such components.